The present invention relates generally to electronic control systems for air conditioners, heat pumps and refrigeration equipment. More particularly, the invention relates to an apparatus and method utilizing digital circuitry to control a heat pump during its normal operating cycle in order to control the accumulation of frost on the outdoor evaporator coil while providing optimum system efficiency and performance during the defrost cycle.
Whether operating as an air-conditioning system or a heat pump system, the basic refrigeration cycle operates by metering refrigerant through a closed system in a precisely controlled manner. The refrigerant cools by evaporation in a heat exchanger commonly called an evaporator coil. The refrigerant is metered to the evaporator coil through an orifice sometimes called an expansion valve. Ideally, a refrigeration system should meter just enough refrigerant into the evaporator coil so that the refrigerant extracts heat throughout the length of the coil as it evaporates. Due to changing dynamics of the system, changes in thermostat settings and changes in load from sun, wind and so forth, the optimal flow through the expansion valve will need to be varied as the system operates.
The precise control of the expansion valve during steady state operation, to achieve optimal efficiency during steady state operation, has been the subject of much study in the continued effort to improve energy efficiency. In addition to the desire to improve efficiency, there is also a need to address the frost buildup problem. In the heating mode operation of a heat pump system, a layer of frost normally builds on the outdoor coil surfaces whenever the temperature of the coil falls below 32.degree. F. This frost significantly reduces the heating capacity of the heat pump and, as the frost continues to accumulate, the performance of the unit eventually degrades to a point where the frost has to be melted.
This defrost process, however, is extremely inefficient. The heating process is interrupted and, in effect, replaced with a counterproductive air conditioning process. This wastes energy and reduces the heating capacity of the heat pump. Heat flows in the wrong direction during defrosting intervals, thus cooling the house or building. To offset this cooling it can be necessary to turn on the resistance heaters, thereby consuming additional energy.
The need to defrost the evaporator coil would appear to be a necessary evil in all but the most arid environments. It would therefore be desirable if a heat pump system could be developed to reduce the number of defrost cycles per day and also to effect the defrosting cycle more quickly and efficiently when it is needed.
The present invention addresses this need through the use of a microprocessor-based control system and digitally controllable expansion valve. The present system utilizes a "demand defrost" scheme wherein the intervals between defrost cycles vary rather than being simply performed on a periodic basis. The defrost cycle is initiated only when a predetermined amount of frost has accumulated on the evaporator coil. This saves energy and improves efficiency by eliminating unnecessary defrost cycles.
In addition, time intervals between defrost cycles are further maximized by making the outdoor coil conditions less favorable to a frost buildup. This is accomplished by monitoring the rate of accumulation of frost buildup in terms of the temperature difference between the ambient air and outdoor coil. If a change in the slope of this temperature difference is sensed, refrigerant flow through the system is increased by modulating the expansion valve setting, thereby raising the outdoor coil temperature. The microprocessor-based system determines the proper valve setting, balancing the objective of maintaining an efficient temperature coil with the objective of gradually raising the temperature of the coil (with some efficiency tradeoff) to retard frost accumulation. The system is therefore able to maintain a level of efficient operation for longer periods.
The present control system also modulates the expansion valve setting to open the valve orifice to a greater diameter than normal on the initiation of the defrost cycle. This enables rapid transfer of the refrigerant charge to the outdoor coil and significantly reduces the length of the defrost cycle. In addition, by pre-starting the outdoor fan prior to termination of the defrost cycle, pressure transients are reduced resulting in greater reliability and longevity of the heat pump and its components.
The result is a state-of-the-art heat pump system which offers high efficiency and longer component life. Because frost buildup on the outdoor coil is monitored and precisely controlled, intervals between defrost can be maximized. Losses incurred by the defrost cycle ar minimized and system efficiency can be optimized.
For a more complete understanding of this invention, its objects and advantages, reference may be had to the following specification and appended claims, taken in conjunction with the accompanying drawings.